Switching amplifiers enjoy significantly better efficiency than their non-switching predecessors, primarily because transistors used to switch voltages to a load are either turned ON, so that the voltage across the transistor is relatively low, or turned OFF, so that the current through the transistor is relatively low. With either a low voltage across the transistor or a low current through the transistor, the power dissipated by the transistor is relatively low.
Switching amplifiers using a single PWM stream are widely used. These switching amplifiers can sometimes generate excessive electromagnetic radio frequency (“RF”) interference that can interfere with the operation of the amplifier as well as with other electronic devices in the vicinity of the amplifier. This RF interference can be attenuated to some extent by coupling the load driven by the amplifier to filters formed by inductors and/or capacitors. However, the remaining RF interference can still be too high in some applications. Other attempts to reduce RF interference using low cost solutions often results in distortion. For instance, reducing the sampling rate of a single PWM stream reduces RF interference, however, it also results in significant distortion, and therefore has not been a viable option. In addition, these switching amplifiers cannot accurately amplify high bandwidth, high accuracy signals at a reasonable cost.
A more recent approach has been to use switching amplifiers capable of using modulation techniques that include two or more PWM streams, such as a multi-reference switching amplifier. A multi-reference switching amplifier is described in U.S. Pat. No. 6,535,058 Multi-reference, High Accuracy Switching Amplifier, the entire content being incorporated herein by reference. One example of a multi-reference switching amplifier comprising two separate PWM streams where one stream is a coarse high voltage PWM stream and the other stream is a fine low voltage PWM stream. Although the multi-referenced switching amplifier resolved the need to amplify high bandwidth, high accuracy signals at a reasonable cost, there is a continued need for improving performance of multi-referenced switching amplifiers, such as reducing RF interference.
Reducing the sampling rate of multi-reference amplifiers by conventional methods, would also result in an increase in distortion to the output signal. In addition, reducing the sampling rate frequency can limit the bandwidth or frequency of the audio signal being amplified. Therefore, there is a need for a system and method for operating switching amplifiers in a manner that reduces electromagnetic RF interference without causing distortion and limiting the bandwidth or frequency of the output signal at a reasonable cost.